Effects of age on the ability of the rat temporomandibular joint to respond to changing functional demands.

This investigation examined the ability of the tissues of the temporomandibular joint (TMJ) to adapt to changing functional demands in young, growing rats compared with mature rats. Functional demands on the TMJ were varied by feeding diets with different physical consistencies. The first group was fed a soft diet for the experimental period. The second group was fed a hard diet, and the third group was initially fed the soft diet, then switched to the hard diet at the mid-point of the experimental period. Gross dimensions of the condyle, mandible, and maxilla were measured with calipers. Thickness of the articular, proliferative, transitional, and hypertrophic zones of the condylar cartilage, and the amount of bone in the subcondylar region and condylar neck were measured on histological sections. Gross dimensions of the condyle were significantly smaller in the soft-diet group compared with the hard- and soft/hard-diet groups in both growing and mature rats. The individual zones of the condylar cartilage were also significantly narrower in the soft-diet group in both growing and mature rats. However, the soft/hard-diet group of mature rats showed only a significant reduction in the thickness of the articular zone of the condylar cartilage compared with the hard-diet group. There were also narrower proliferative and transitional zones in the mature rats fed a soft/hard diet. In contrast, all of these zones showed full recovery in the young rats fed a soft/hard diet. The data presented here suggest that increasing age may diminish the capacity of the TMJ to adapt to altered function and consequently may play a significant role in the development of degenerative joint disease.